DNN uses the default ASP.NET membership processes so you can do this quite easily if you just want to allow the same login information to be used by both sites.

First of all you will want both to use the same database, as they must both have access to the aspnet_ tables.  Secondly, do NOT use an object qualifier in your  DNN installation, I have found that this causes problems.

Now, you will then just need to configure the asp.net membership provider for both applications to have the same application name and type for storage of passwords.  (DNN default is encrypted).  The final step that you must do is to ensure that the machine keys (Validation and Decryption) are the same for both application.